In the Graduate College dining room at Princeton everybody used to sitwith his own group. I sat with the physicists, but after a bit I thought: Itwould be nice to see what the rest of the world is doing, so I'll sit for aweek or two in each of the other groups. When I sat with the philosophers I listened to them discuss veryseriously a book called Process and Reality by Whitehead. They were usingwords in a funny way, and I couldn't quite understand what they were saying.Now I didn't want to interrupt them in their own conversation and keepasking them to explain something, and on the few occasions that I did,they'd try to explain it to me, but I still didn't get it. Finally theyinvited me to come to their seminar. They had a seminar that was like a class. It had been meeting once aweek to discuss a new chapter out of Process and Reality -- some guy wouldgive a report on it and then there would be a discussion. I went to thisseminar promising myself to keep my mouth shut, reminding myself that Ididn't know anything about the subject, and I was going there just to watch. What happened there was typical -- so typical that it was unbelievable,but true. First of all, I sat there without saying anything, which is almostunbelievable, but also true. A student gave a report on the chapter to bestudied that week. In it Whitehead kept using the words "essential object"in a particular technical way that presumably he had defined, but that Ididn't understand. After some discussion as to what "essential object" meant, theprofessor leading the seminar said something meant to clarify things anddrew something that looked like lightning bolts on the blackboard. "Mr.Feynman," he said, "would you say an electron is an 'essential object'?" Well, now I was in trouble. I admitted that I hadn't read the book, soI had no idea of what Whitehead meant by the phrase; I had only come towatch. "But," I said, "I'll try to answer the professor's question if youwill first answer a question from me, so I can have a better idea of what'essential object' means. Is a brick an essential object?" What I had intended to do was to find out whether they thoughttheoretical constructs were essential objects. The electron is a theory thatwe use; it is so useful in understanding the way nature works that we canalmost call it real. I wanted to make the idea of a theory clear by analogy.In the case of the brick, my next question was going to be, "What about theinside of the brick?" -- and I would then point out that no one has everseen the inside of a brick. Every time you break the brick, you only see thesurface. That the brick has an inside is a simple theory which helps usunderstand things better. The theory of electrons is analogous. So I beganby asking, "Is a brick an essential object?" Then the answers came out. One man stood up and said, "A brick as anindividual, specific brick. That is what Whitehead means by an essentialobject." Another man said, "No, it isn't the individual brick that is anessential object; it's the general character that all bricks have in common-- their 'brickness' -- that is the essential object." Another guy got up and said, "No, it's not in the bricks themselves.'Essential object' means the idea in the mind that you get when you think ofbricks." Another guy got up, and another, and I tell you I have never heard suchingenious different ways of looking at a brick before.